Various optical devices are known which employ prisms upon which a finger whose print is to be identified is placed. The prism has a first surface upon which a finger is placed, a second surface disposed at an acute angle to the first surface through which the fingerprint is viewed and a third illumination surface through which light is directed into the prism. In some cases, the illumination surface is at an acute angle to the first surface, as seen for example, in U.S. Pat. Nos. 5,187,482 and 5,187,748. In other cases, the illumination surface is parallel to the first surface, as seen for example, in U.S. Pat. Nos. 5,109,427 and 5,233,404. Fingerprint identification devices of this nature are generally used to control the building-access or information-access of individuals to buildings, rooms, and devices such as computer terminals.
One of the problems associated with fingerprint sensors concerns the reliable and accurate transformation of ridge and valley pattern of the fingertip into electrical or optical signals to be stored in a digital format. Optical systems as described above, for example using a prism, require sophisticated equipment and tend to be bulky and costly.
In an attempt to overcome some of the limitations and disadvantages of using optical systems based on illumination of the finger tip, U.S. Pat. No. 4,353,056 in the name of Tsikos issued Oct. 5, 1982, discloses an alternative kind of fingerprint sensor that uses a capacitive sensing approach. The described sensor has a two dimensional, row and column, array of capacitors, each comprising a pair of spaced electrodes, carried in a sensing member and covered by an insulating film. The sensors rely upon deformation to the sensing member caused by a finger being placed thereon so as to vary locally the spacing between capacitor electrodes, according to the ridge/trough pattern of the fingerprint, and hence, the capacitance of the capacitors. In one arrangement, the capacitors of each column are connected in series with the columns of capacitors connected in parallel and a voltage is applied across the columns. In another arrangement, a voltage is applied to each individual capacitor in the array. Sensing in the respective two arrangements is accomplished by detecting the change of voltage distribution in the series connected capacitors or by measuring the voltage values of the individual capacitances resulting from local deformation. To achieve this, an individual connection is required from the detection circuit to each capacitor.
While the described sensor may not suffer from the problems associated with the kind of sensor employing an optical sensing technique, it suffers from its own problems. For example, applying a voltage to the array of capacitors requires circuitry to each capacitor for charging. Such charging also requires further states in the imaging process consuming more resources and providing added areas for unreliability. Moreover, the need to provide a respective connection to each individual capacitor in the array means that a very large number of connecting lines is necessary. This creates difficulties, both in the fabrication of the sensing member and its interconnection with the detection circuit.
In yet another attempt to improve upon deficiencies and limitations of the aforementioned and other prior art, a further contact imaging device is described in U.S. Pat. No. 5,325,442 in the name of Knapp, issued Jun. 28, 1994. Those parts of the disclosure of this patent not included in this specification are incorporated herein by reference.
Knapp describes making a capacitance measuring imaging device in the form of a single large active matrix array involving deposition and definition by photolithographic processes of a number of layers on a single large insulating substrate. Electrodes and sets of address conductors formed of metal and field effect transistors are formed as amorphous silicon or polycrystalline silicon thin film transistors (TFTs) using an appropriate substrate of, for example, glass or quartz.
Although Knapp attempts to provide an improvement over Tsikos mentioned above, other disadvantages and limitations become evident in the manufacture implementation of Knapp's disclosed device. Firstly, it is extremely difficult to produce a single large imaging contact device, for example comprised of a single silicon die cut from a silicon wafer. Fabricating a device with a contact area of 0.75 inches by 0.75 inches or larger, approximately a required dimension for imaging a fingerprint, is impractical due to the fragile nature of silicon devices. Aside from large dies being costly to manufacture, they have lower manufacturing yields than smaller dies. When square or rectangular dies are cut from a substantially round silicon wafer, there is less loss at the edges of the wafer when small dies are cut. The mechanical strength of these chips also limits their use in contact applications; for instance, the force of a finger contacting and resting upon a large die can cause a crack or stress fracture. Furthermore, current, conventional photolithographic systems are typically equipped for the production of dies that have a maximum dimension of about 0.4 inches to 0.5 inches.
Two-dimensional arrays used for capacitive imaging of fingerprints are expensive, subject to cracking when used over a period of time and so forth. This fragility and cost limits the widespread use of capacitive imaging of fingerprints.
A fingerprint sensing device and recognition system that includes an array of closely spaced apart sensing elements each comprising a sensing electrode and an amplifier circuit is described in U.S. Pat. No. 5,778,089 in the name of Borza, issued Jul. 7, 1998. The device is used to sense electrical charge on a fingertip and obviates the need to pre-charge the sensing electrode. The device may be constructed with a single die or with multiple dies. Those parts of the disclosure of this patent not included in this specification are incorporated herein by reference.
In order to reduce the overall size of a capacitive fingerprint imager, it would seem necessary to reduce the overall imaged area. Unfortunately, because humans are not precise in finger placement, this makes identification difficult or impossible. A smaller capacitive fingerprint scanner would be advantageous due to lower manufacturing costs, improved robustness, and so forth. Also, the small area required is highly advantageous for embedded applications such as with a cell phone, a telephone, a computer (laptop) and so forth.